1. Field of the Invention
The present invention relates to an (electric) motor having permanent magnets in a rotor, in particular, to a motor in which magnetic-field characteristics of the permanent magnets in the rotor are variable.
Priority is claimed on Japanese Patent Application No. 2006-176296, filed Jun. 27, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
In a known example of such a motor, an inner-peripheral rotor and an outer-peripheral rotor, each individually having permanent magnets, are arranged coaxially, and they are relatively rotated along a circumferential direction (i.e., the relative phase between the rotors is changed), so that the magnetic-field characteristics of the whole rotor are variable (see, for example, Japanese Unexamined Patent Application, First Publication No. 2002-204541).
In this motor, in order to vary the relative phase between the inner-peripheral and outer-peripheral rotors in accordance with the rotation speed of the motor, one of the rotors is rotated with respect to the other along the circumferential direction, by using a member which is displaced along a radial direction due to centrifugal force. On the other hand, in order to vary the relative phase between the inner-peripheral and outer-peripheral rotors in accordance with the rotation speed of the rotational magnetic field generated in a stator, relative positions of the rotors along the circumferential direction are changed by applying a control current to stator windings while each rotor maintains the relevant rotation speed by means of inertia.
In the above-described motor, when the permanent magnets of the outer-peripheral and inner-peripheral rotors are made to face each other in a manner such that opposite poles (i.e., N-pole and S-pole) face each other (i.e., each rotor has the same pole-position setting), the magnetic field of the whole rotor is strengthened, and induced voltage is increased. In contrast with this, when the permanent magnets of the outer-peripheral and inner-peripheral rotors are made to face each other in a manner such that the same poles (i.e., N-poles or S-poles) face each other (i.e., opposite pole-position settings are selected), the magnetic field of the whole rotor is weakened, and induced voltage is decreased.
However, in the above conventional motor, conditions for varying the relative phase between the outer-peripheral and inner-peripheral rotors are limited, and it is impossible to freely vary the relative phase when the motor is stopped or involuntarily rotated. In particular, when such a motor is used for driving a hybrid vehicle or an electrically-driven vehicle, it is required to instantly change the characteristics of the motor, in accordance with the driving state of the vehicle, to desired ones. In order to satisfy such a requirement, it is important to improve the degree of freedom in the control for varying the relative phase.